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United States Patent |
5,223,452
|
Knepprath
|
June 29, 1993
|
Method and apparatus for doping silicon spheres
Abstract
A method and apparatus for doping silicon spheres (48) with a solid
phosphorous source (41, 42) is disclosed. Two solid sheets (41, 42) of
solid phosphorous source are held in a chamber (32) and aligned
substantially parallel to one another for holding the plurality of silicon
spheres therebetween. The chamber (32) temperature is increased to
vaporize the sheets (41 and 42) to evenly diffuse phosphorous vapor into
the silicon spheres (48).
Inventors:
|
Knepprath; Vernon E. (4904 W. Park Ave., Sherman, TX)
|
Appl. No.:
|
784614 |
Filed:
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October 29, 1991 |
Current U.S. Class: |
438/565; 257/E21.141; 257/E31.039; 438/567; 438/568 |
Intern'l Class: |
H01L 021/223 |
Field of Search: |
437/165,168,169,160
156/620.4
|
References Cited
U.S. Patent Documents
3986905 | Oct., 1976 | Garavaglia | 437/954.
|
3998659 | Dec., 1976 | Wakefield | 437/169.
|
4661177 | Apr., 1987 | Powell | 437/160.
|
4789596 | Dec., 1988 | Allen et al. | 156/620.
|
Foreign Patent Documents |
0154268 | Dec., 1979 | JP | 437/168.
|
0020510 | Feb., 1985 | JP | 437/168.
|
0196816 | Aug., 1989 | JP | 437/168.
|
Primary Examiner: Hearn; Brian E.
Assistant Examiner: Dang; Trung
Attorney, Agent or Firm: Burke; Richter Darryl, Kesterson; James C., Donaldson; Richard L.
Parent Case Text
This application is a continuation of application Ser. No. 454,326, filed
Dec. 12, 1989 , now abandoned.
Claims
What is claimed is:
1. A method for doping a plurality of silicon spheres with a dopant, which
comprises the steps of:
positioning said plurality of silicon spheres in a chamber;
utilizing a solid dopant source to provide a contact and support area for
said silicon spheres; and
vaporizing dopant from said solid dopant source to create a dopant cloud,
simultaneously diffusing dopant directly from said solid dopant source
through said support area and from said dopant cloud into said silicon
spheres causing substantially even diffusion of phosphorus into said
silicon spheres.
2. The method as recited in claim 1, wherein dopant comprises solid
phosphorous source.
3. The method as recited in claim 2, wherein said step of vaporizing
comprises the step of heating said chamber to induce vaporization of said
solid phosphorous source.
4. The method as recited in claim 3, wherein said heating step increases
said chamber temperature to approximately 950.degree. C. for approximately
30 minutes.
5. The method as recited in claim 1, further comprising the steps of
aligning two solid phosphorous sheet sources apart from one another for
holding the silicon spheres therebetween; and
vaporizing dopant from said two solid phosphorous sheet sources to create a
dopant cloud, simultaneously diffusing dopant directly from said two solid
phosphorous sources through said support area and from said dopant cloud
into said silicon spheres causing substantially even diffusion of
phosphorous into said silicon spheres.
6. The method as recited in claim 1, further comprising a step of evenly
distributing a plurality of substantially spherical phosphorous source
pieces with the silicon spheres.
7. The method as recited in claim 1, further comprising a step of inserting
a boat having a phosphorus source layer deposited thereon into said
chamber for holding the silicon spheres.
8. The method as recited in claim 7, further comprising a step of inserting
a top having a phosphorus source layer deposited thereon, whereby said top
is placed over said silicon spheres to enhance diffusion within said
spheres.
9. The method as recited in claim 8, wherein said top contacts said silicon
spheres.
10. The method as recited in claim 1, wherein said silicon spheres are not
in contact with one another.
11. The method as recited in claim 1, wherein said silicon spheres form a
single layer.
12. The method as recited in claim 1, wherein said solid dopant source is
in particulate form.
13. The method as recited in claim 1, wherein said silicon spheres and said
solid dopant source are intimately mixed.
Description
TECHNICAL FIELD OF THE INVENTION
This invention relates generally to a method and apparatus for doping
silicon spheres, and more particularly to a method and apparatus for
doping silicon spheres for use in solar arrays.
BACKGROUND OF THE INVENTION
A system that has proven useful for efficiently producing electricity from
the sun's radiation is described in U.S. Pat. No. 4,691,076, which is
assigned to the present assignee. In that system, an array is formed of
semiconductor spheres, each sphere having a P-type interior and an N-type
skin. The plurality of silicon spheres are housed in a pair of aluminum
foil members which form the contacts to the P-type and N-type regions.
Multiple arrays are interconnected to form a module of solar cell elements
for converting sunlight into electricity.
In order to produce efficient quantities of arrays, it is desireable to
have a process which is uncomplicated, efficient and cost effective. A key
process step in making the solar arrays which has created difficulties is
the introduction of controlled quantities of dopant impurity atoms into
the silicon spheres of the solar arrays.
In one previously developed method of introducing a phosphorous dopant into
the silicon spheres, the dopant is delivered to the surface of the spheres
in a vapor-phase at the proper concentration. While this method works
relatively well for doping planar silicon surfaces, it tends to be
ineffective when applied to spherical bodies, such as those silicon
spheres used for solar cells. For example, it is difficult to obtain
uniform diffusion depth within the silicon spheres where two spheres have
a point-to-point contact. At these point-to-point contacts, the spheres
are shielded from the dopant vapor which results in a nondiffused area and
causes electrical shorts in the spheres. Consequently, the entire sphere
surface must be doped. Additionally, electrostatic problems can result in
the vapor-phase to cause silicon spheres to cluster together which causes
nonuniform diffusion.
Some experimentation has been performed in the application of a liquid
dopant source such as a liquid phosphorous. Generally, this method has
been referred to as a spin-on phosphorous dopant which uses a liquid to
attempt to evenly coat the silicon sphere. Unfortunately, when using a
liquid dopant, it has been very difficult to apply a uniform coating due
to the point-to-point contact between the silicon spheres. It has been
found that if the liquid is too thin, the silicon spheres tend to form
pinholes along its surface. Conversely, if the liquid dopant is too thick,
the dopant film on the silicon spheres tends to crack.
Therefore, there is a need for a method which evenly dopes a silicon sphere
to create a uniform diffusion along the entire surface of the silicon
sphere. Additionally, a need has arisen for a method of doping which will
eliminate prior problems associated with the point-to-point contact of
silicon spheres. There is also a need for a method which will reduce the
number of processing steps associated with phosphorous doping of silicon
spheres. Finally, a need exists for a method of doping a plurality of
silicon spheres used in solar cells which is cost effective.
SUMMARY OF THE INVENTION
In accordance with the present invention, a method and apparatus are
provided for doping silicon spheres with a solid phosphorous source for a
solar cell. The apparatus of the present invention generally comprises a
quartz chamber having a furnace attached to its circumference. The silicon
spheres are held between two sheets of solid phosphorous source such that
the sheets are substantially parallel. These sheets are separated by a
resistivity pilot to keep the silicon spheres from touching the top
phosphorous sheet. In order to produce a uniform diffusion within the
silicon spheres, the chamber interior temperature is increased to
approximately 950 degrees Centigrade (.degree.C.) for approximately 30
minutes. This heating causes the solid phosphorous source sheets to
vaporize and evenly diffuse into the silicon spheres.
In an alternative embodiment, the apparatus of the present invention
comprises a quartz boat held in the chamber for receiving the plurality of
silicon spheres. A plurality of substantially spherical pieces of solid
phosphorous source are randomly distributed on the boat with the silicon
spheres such that there is an even distribution of the spheres and the
pieces. Once there has been an even distribution, the chamber is heated to
approximately 950.degree. C. with a furnace to vaporize the pieces of
solid phosphorous source, such that the vapor diffuses into the silicon
spheres.
Another alternative embodiment of the present invention comprises a quartz
boat having a even layer of phosphorous source deposited thereon. When the
temperature of the chamber is increased to approximately 950.degree. C.,
the phosphorous source layer vaporizes and diffuses into the silicon
spheres.
The present invention presents several technical advantages over
conventional processes used for doping silicon spheres with a phosphorous
dopant. The present invention features a solid phosphorous source used for
doping silicon spheres which is less cumbersome than prior art. When
vaporized, the solid phosphorous source evenly diffuses into the silicon
spheres. This even distribution of the vapor allows for diffusion despite
the point-to-point contact of silicon spheres. The present invention is
capable of adequately doping silicon spheres with a phosphorous dopant,
while reducing the number of processing steps and expense associated with
the prior art.
BRIEF DESCRIPTION OF THE DRAWINGS
Other aspects of the invention and their advantages will be discerned after
studying the Detailed Description in conjunction with the Drawings in
which:
FIG. 1 is a cross-sectional view of an apparatus as used in prior art for
diffusing a phosphorous gas into a plurality of silicon spheres;
FIG. 2 is a cross-sectional view of the apparatus in accordance with the
present invention, illustrating a plurality of silicon spheres held
between two sheets of solid phosphorous source;
FIG. 3 is a top view of the apparatus as seen along lines 3--3 in FIG. 2;
FIG. 4 is a cross-sectional view of the apparatus as shown in FIG. 2,
illustrating an alternative method of holding the plurality of silicon
spheres onto the sheet of solid phosphorous source;
FIG. 5 is a cross-sectional view of an alternative apparatus used for
doping silicon spheres having a plurality of substantially spherical
phosphorous sources evenly distributed with the plurality of silicon
spheres;
FIG. 6 is a cross-sectional view of another alternative apparatus used for
doping silicon spheres having a plurality of phosphorous chunks evenly
distributed with the plurality of silicon spheres; and
FIG. 7 is a cross-sectional view of another alternative embodiment having a
boat lined with a phosphorous source layer for doping the silicon spheres
held therein.
DETAILED DESCRIPTION OF THE INVENTION
Referring initially to FIG. 1, a cross-sectional view of an apparatus as
used in prior art can be seen. Specifically, an apparatus generally
designated 10 can be seen which comprises a chamber 12 having a furnace 14
contained thereon for heating chamber 12 during processing. Chamber 12 has
an inlet port 16 at one end for connecting to an inlet line 18.
Inlet line 18 is used for allowing a gas source 20 to enter therethrough.
Contained within chamber 12 is a boat 22 which can be held in place by a
base 24 which is connected to legs 26. A plurality of silicon spheres 28
are evenly distributed along boat 22 for subsequent doping of silicon
spheres 28. In accordance with the prior art, gas source 20, which is
preferably a mixture of phosphorous vapor and an oxygen gas, is brought
through inlet line 18 into the interior of chamber 12. Unfortunately, the
diffusion of gas source 20 is not evenly dispersed to silicon spheres 28
because of a plurality of point-to-point contacts 29 between spheres 28.
An outlet line 29 is provided for exhausting gas.
Referring now to FIG. 2, the preferred embodiment of the present invention
can be seen. An apparatus which is generally designated 30 is shown having
a chamber 32 with a furnace 34 attached to its circumference. Chamber 32
is preferably made of a quartz material. Chamber 32 has an inlet port 36
at one end for connecting to an inlet line 38. Inlet line 38 is used for
allowing a gas source 40 to enter therethrough. The interior of chamber 32
contains two sheets of solid phosphorous sources 41 and 42. Top sheet 41
and bottom sheet 42 are substantially parallel such that a plurality of
silicon spheres 48 are held therebetween. In order to keep top sheet 41
from contacting silicon spheres 48, a plurality of resistivity pilots 44
are held between sheets 41 and 42. Sheets 41 and 42, spheres 48, and
pilots 44 are held within chamber 32 by a base 46.
In operation, the temperature of chamber 32 is increased by applying energy
to furnace 34. Additionally, gas source 40 is injected into inlet line 38
and circulated through chamber 32. Gas source 40 is preferably an inert
gas, such as nitrogen, used to prevent back flow of contaminants into
chamber 12. Once the temperature of chamber 32 is increased to
approximately 950.degree. C., solid source phosphorous sheets 41 and 42
vaporize to create a cloud of phosphorous vapor within chamber 32. The
specific reaction is illustrated by:
SiP.sub.3 O.sub.7 .fwdarw.SiO.sub.2 +P.sub.2 O.sub.5
As can be appreciated, this reaction is accelerated by increasing the
temperature of chamber 32. Once the vaporization of phosphorus sheets 41
and 42 has occurred, the vapor will evenly diffuse into silicon spheres
48. Because the vapor comes from two sides of silicon spheres 48, the
diffusion within silicon spheres 48 is uniform at point-to-point contacts
49. An outlet line 51 is provided for exhausting a gas.
FIG. 3 illustrates a top view of apparatus 30 as seen along the line 3--3
of FIG. 2. Phosphorus sheet 42 has a plurality of separators 50 held
thereon. Separators 50 function to position silicon spheres 48 on sheet
42. Resistivity pilots 44 are disposed at opposing corners of sheets 41
and 42, and are used for insuring that portions of spheres 48 are not in
contact with phosphorous source sheet 41.
Referring now to FIG. 4, an alternative embodiment of the invention can be
seen. Specifically, sheet 42 has a plurality of grooves 52 formed therein
for holding silicon spheres 48. When sheet 42 is vaporized, there is a
higher probability that there will be even diffusion into spheres 48
because of the separation between spheres 48. This embodiment insures
elimination of point-to-point contacts 49 between silicon spheres 48 as
illustrated in FIG. 2.
Referring to FIG. 5, an alterntive embodiment of the present invention can
be seen. FIG. 5 is similar to the embodiment of FIG. 2 with the exception
of a boat 54 which is held on base 46. The plurality of silicon spheres 48
are held within boat 54, which prreferably comprises a quartz material.
Also held within boat 54 are a plurality of substantially spherical solid
phosphorous source pieces 56 which are distributed throughout boat 54 such
that there is contact between pieces 56 and spheres 48. In operation, when
the interior of chamber 32 is increased to approximately 950.degree. C.,
pieces 56 vaporize and diffuse within silicon shperes 48. Because of the
even distribution of pieces 56, this embodiment is capable of insuring a
reduction in point-to-point contacts between silicon spheres 48.
Referring now to FIG. 6, another alternative embodiment of the present
invention can be seen. FIG. 6 illustrates an embodiment similar to that of
FIG. 5. Specifically, a plurality of phosphorous source chunks 56 are
evenly distributed throughout boat 54 such that there is contact between
chunks 56 and silicon spheres 48. This alternative embodiment has the
advantage of containing larger solid phosphorous source chunks 56 which
are easily handled during operation by the user.
Referring now to FIG. 7, another alternative embodiment of the present
invention can be seen. Specifically, boat 54 has a phosphorous source
layer 60 evenly deposited thereover the upper surface thereof. When the
temperature of the chamber 32 increases to approximately 950.degree. C.,
phosphorous source layer 60 will vaporize and diffuse into silicon spheres
48. This alternative embodiment is equipped with a top 62 which can be
optionally used. Top 62 preferably has a thin layer of solid phosphorous
source evenly coated thereon to enhance the extent of diffusion within
silicon spheres 48. This particular embodiment is able to capture the
vapor within a region 64 which is created between top 62 and boat 54. Boat
54 and top 62 can be made of a quartz material. Additionally, boat 54 and
top 62 can be relayered with a phosphorous source for allowing them to be
recycled and reused for doping silicon spheres 48.
The present invention can be illustrated by reference to the examples
below:
EXAMPLE 1
The preferred embodiment, which features two sheets of a solid phosphorous
source having silicon spheres contained therein, was tested. The sheets
were manufactured by SOHIO Engineered Materials Company and are commercial
known as PDS Phosphorous PH-950 N-type planar diffusion sources. When a
diffusion temperature of approximately 950.degree. C. was applied for
approximately 30 minutes to the active component (PH-950), SiP.sub.3
O.sub.7 decomposed to form the desired dopant series, P.sub.2 O.sub.5
vapor. The silicon spheres had a diameter of approximately 20 mils.
The silicon spheres were assembled to form arrays used in a solar cell and
the solar cell was tested for its integrity. The test revealed that the
open circuit voltage was approximately 548 millivolts. The short circuit
current of the solar cell was approximately 1.3 miliamps. The fill factor
associated with the solar cell had a 72.3% effectiveness. The overall cell
efficiency was approximately 6.2% which proved to be very beneficial for
utilization as a solar cell in the industry. Each silicon sphere of the
solar arrays was tested for its integrity to determine if adequate
phosphorous diffusion had been performed on all spheres. It was determined
that each of the 37 spheres had acceptable integrity for use in a solar
cell. It was also determined that the concentration of the carrier was
approximately 1.0.times.10.sup.17 atoms per centimeter cubed
(atoms/cc.sup.3) at a depth within each sphere between approximately 0.5
microns to approximately 2.0 microns. Surface concentration of the silicon
sphere is 3.0.times.10.sup.20 atoms/cc.sup.3.
EXAMPLE 2
A test was performed utilizing a plurality of substantially spherical
phosphorous pieces evenly distributed throughout the silicon spheres. The
silicon spheres had an average diameter of 20 mils. These pieces of
phosphorous had an approximate diameter of 10 microns. Once the furnace
was increased to 950.degree. C. for approximately 30 minutes, the
diffusion proved to be successful throughout all silicon spheres. The
solar cell produced from the silicon spheres was tested and the open
circuit voltage was approximately 560 millivolts and the short circuit
current was approximately 1.68 miliamps. The fill factor efficiency was
approximately 68.2%. The overall solar cell efficiency was 8.6%. The
concentration of phosphorous was approximately 1.0.times.10.sup.17
atoms/cc at a depth within each sphere between approximately 0.5 microns
to approximately 2.0 microns. Surface concentration of the silicon sphere
is 3.0.times.10.sup.20 atoms/cc.sup.3.
EXAMPLE 3
A test was conducted using a boat for holding the silicon spheres having a
phosphorous layer deposited thereon with a top for holding the vapor
between the region formed therebetween. The silicon spheres had an average
diameter of 20 mils. The temperature of the furnace was increased to
approximately 950.degree. C. and held for approximately hour. The open
circuit voltage of the solar cell produced from these silicon spheres was
562 miliamps and the short circuit current was approximately 1.7 miliamps.
The fill factor for the solar cell was approximately 6.9%, while the
overall cell efficiency was 8.7%. The concentration of phosphorous was
approximately 1.0.times.10.sup.17 atoms/cc at a depth within each sphere
between approximately 0.5 microns to approximately 2.0 microns. Surface
concentration of the silicon sphere is 3.0.times.10.sup.20 atoms/cc.sup.3.
In summary, an advantageous method for doping silicon spheres has been
disclosed which features the use of a solid phosphorous source held within
a chamber. This phosphorous source is vaporized by increasing the furnace
temperature to evenly diffuse into the silicon spheres. This method of
doping has been useful for doping silicon spheres used in solar cells. The
diffusion has proven to be uniform throughout the entire sphere to avoid
the prior problems associated with point-to-point contacts between the
spheres.
While the preferred embodiments of the invention and their advantages have
been disclosed in the above-detailed Description, the invention is not
limited thereto, but only by the spirit and scope of the appended claims.
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